Patent Literature 1 discloses a heat exchanger in which gas passages for gas to flow and liquid passages for liquid to flow are stacked. As shown in FIG. 25, an exhaust-gas heat exchanging device 100 disclosed in Patent Literature 1 includes an exterior case 101, a plurality of tubes 110 housed inside the exterior case 101, and a pair of tanks 120, 121 disposed at the opposite ends of the plurality of tubes 110.
The exterior case 101 is provided with a cooling-water inlet 102 and a cooling-water outlet 103 for cooling water (cooling fluid). Inside exterior case 101, cooling water passages 104 are formed of the gaps between the adjacent tubes 110 and the like.
The opposite ends of all the tubes 20 are open to the inside of the pair of tanks 120, 121. One of the tanks, namely, the tank 120, is provided with an exhaust-gas inlet portion 120a while the other tank 121 is provided with an exhaust-gas outlet portion 121a.
The tubes 110 are stacked on top of one another. As shown in FIG. 26, each tube 110 is formed of two flat members 110a, 110b. An exhaust gas passage 111 is formed inside each tube 110. A fin 112 is disposed in the exhaust gas passage 111.
As shown in FIG. 27, the fin 112 is formed in a rectangular wave shape when viewed from the upstream side in the exhaust-gas flow direction S. The fin 112 includes a plurality of protruding pieces 113 that are lanced at intervals in the exhaust-gas flow direction S. Each protruding piece 113 has a triangular shape and protrudes in such a way as to impede the flow of the exhaust gas inside the exhaust gas passage 111. The orientation angle of the protruding piece 113 is oblique to a direction perpendicular to the exhaust-gas flow direction S.
The exhaust gas from an internal combustion engine flows through the exhaust gas passage 111 inside each tube 110. The cooling water flows through each cooling water passage 104 inside the exterior case 101. The exhaust gas and the cooling water exchange heat with each other through the tube 110 and the fin 112. In this heat exchange, the protruding pieces 113 of the fin 112 disturb the flow of the exhaust gas, thereby promoting the heat exchange.
As shown in FIG. 28, the exhaust gas flowing through the exhaust gas passage 111 cannot flow straight due to each protruding piece 113, and a low pressure region is therefore formed immediately downstream of (behind) the protruding piece 113. As shown in Parts (a) and (b) of FIG. 29, the exhaust gas colliding with the protruding piece 113 flows over oblique sides 113a, 113b of the protruding piece 113 and comes around behind the protruding piece 113. Due to the triangular shape of the protruding piece 113 (the slopes by the oblique sides 113a, 113b), each of a first flow that flows over the oblique side 113a and a second flow that flows over the oblique side 113b is such that the flow rate is large on the upper side of the slope and the flow rate is small on the lower side of the slope. Rotating force is exerted on each of the first flow and the second flow when the flow with the above flow rate distribution is drawn into the low pressure region mentioned above. As a result, as shown in Part (c) of FIG. 29, the first flow and the second flow each become a vortex flow. Thus, two vortex flows are formed downstream of the protruding piece 113. These vortex flows flow while disturbing a boundary layer formed in the vicinity of the inner surface of the exhaust gas passage 111 (exhaust-gas stationary layer). Hence, the heat exchange rate is improved.